Joey and Igor are right—let the installer run the fiber cable for you. It’s their job, it doesn’t cost you anything, and that way, if a technical issue develops in the fiber or the ONT, it’s Sonic’s responsibility.
Sonic expects the installation to take about 2 hours (I believe), so your job is to provide the installer with a quick and easy fiber route from the transition box to your computer room. Accessible crawl space is good. Conduit, including ENT (flexible, corrugated conduit), also works, if you have it in place.
If you plan to route the fiber in the stud cavity without conduit—for instance, from the sub-area, up through the floor and the stud cavity, to a spot on the wall of your computer room—plan to pre-drill the holes in the floor or ceiling, the wall, and any blocks or studs, and run a pull string (or cable) through the holes along the route so the installer can pull the fiber into place. When sizing and placing your holes, be aware that the secondary fiber cable is roughly the same size as Cat5e (though flatter), but it’s springier and stiffer.
Good luck with your preparations.